Piezo-electric tag

ABSTRACT

A piezo-electric tag in the form of a card has a first dipole antenna, a first rectification circuit, a piezo-electric transformer, a second rectification circuit, and a transponder circuit. In operation, the antenna receives incoming radiation and generates a corresponding signal which propagates to the first circuit which demodulates and filters it to generate a signal which is applied to the transformer to excite it. The transformer increases the voltage amplitude of the signal by generating a relatively higher voltage amplitude signal which is used in the tag to generate a signal for supplying power to the transponder. The transformer provides voltage magnitude enhancement to generate potentials suitable for operating active electronic circuits incorporated into the tag. The tag can be personnel wearable and even adapted for permanent inclusion into biological systems.

This application is a 371 national stage entry of PCT/GB00/02944 datedJul. 31, 2000, which claims priority to United Kingdom application9917856.8 dated Jul. 29, 1999.

The present invention relates to a piezo-electric tag.

BACKGROUND OF THE INVENTION

Tags are portable devices which are capable of being attached to itemsor personnel wearable. They can be used, for example, for remotelyidentifying the items or receiving information therefrom. In manyapplications, the tags must be compact and be capable of respondingafter long periods of inactivity, for example where the tags areincorporated into items placed into storage for periods of severalyears.

Conventionally, tags can be passive devices which modify and reflectinterrogating radiation directed thereto from associated interrogatingsources. Because the tags do not provide power gain, their operatingrange from the sources is often limited to a few meters.

Active tags are known which incorporate onboard power sources such as aminiature electrical cell. Such power sources have a limited operatinglifetime, especially if they are required to power their associated tagscontinuously. Moreover, the sources can make the tags unacceptably bulkyfor some applications, for example where tags are implemented as filmstrips for incorporating into spines of library books.

Although it is feasible to power tags from radiation incident thereupon,for example using solar cells incorporated into the tags or byinductively coupling energy from associated interrogating sources to thetags, it is not practicable in some circumstances to do this for safetyreasons, for reasons of restricted operating range or for reasons ofobscuration in the case of solar cells.

The use of received radio radiation for powering electronic tags isknown in the art, for example as disclosed in a published patentapplication no GB 2 306 081A. In the application, there is described apassive electrical power supply for providing electrical power to anelectronic tag, the supply comprising an antenna for converting receivedradio frequency radiation into a first electrical signal, and atransformer including wire-wound coils for transforming the first signalinto a second signal capable of altering the impedance of a field effecttransistor (FET). In operation, the FET provides at its drain electrodea quasi half-wave rectified representation of the second signal which isconverted to a unipolar signal by a capacitor connected to the drainelectrode, the unipolar signal providing a power supply potential foroperating the tag. The supply is operable to convert the receivedradiation into the unipolar signal such that the transformer operates atthe frequency of the received radiation received at the antenna. Thetransformer can optionally be an autotransformer comprising a singlewire-wound coil.

A power supply for a transponder is also disclosed in a published patentapplication no. GB 2 303 767 A. The supply described provides power to aresponse circuit of the transponder, the supply generating directcurrent (d.c.) from received electromagnetic energy. The supplycomprises a capacitor charged from a rectifier diode, the diode having acharacteristic such that its reverse resistance against a reversecurrent directed at its n region to its p region is lower than itsforward resistance against a reverse current directed from its p regionto its n region. The diode is thus connected reversely compared to aconventional diode, its anode being connected to a positive plate of thecapacitor. The arrangement allows the transponder to remain functionaleven when the received electromagnetic energy is relatively weak. Therequired characteristic for the diode can be implemented by theavalanche or tunnel effect. Moreover, a voltage multiplier may beprovided by using a plurality of the diodes with associated capacitorsfor generating higher supply potentials. The supply does not employ anyform of transformer for increasing the potential of signals generated inresponse to receiving the electromagnetic energy.

Piezo-electric transformers capable of stepping up potentials are alsoknown in the art, for example as described in U.S. patent nos. U.S. Pat.No. 5,828,160 and U.S. Pat. No. 5,389,852. Such transformers areoperable to resonate at a frequency typically in a range of several tensof kHz to 300 kHz when stepping up potentials. This range of frequenciesis considerably less than that used for electromagnetic radiationconventionally employed to interrogate electronic tags, for example 10MHz to 30 GHz. Although piezoelectric transformers operating atfrequencies above 300 kHz can be fabricated for example 600 z, theircost and difficulty of fabrication renders them unattractive for itemssuch as electronic tags.

Non-contact energy coupling schemes employing piezo-electric devices areknown in other technical fields, for example as disclosed in a U.S. Pat.No. 5,749,909 concerning medically implanted devices. In the patent,there is described an energy transmission system for transmitting energynon-invasively from an external unit to an implanted medical device torecharge a battery in the medical device. An alternating magnetic fieldis generated by the external unit and a piezo-electric device in theimplanted medical device vibrates in response to the magnetic flux togenerate a voltage. The voltage is rectified and regulated to providecharging current to a rechargeable battery in the medical device. In thearrangement, the piezoelectric device is stimulated by the magnetic fluxat a resonant frequency of the device, namely in the order of tens ofkHz.

The inventor has appreciated that a principal problem associated withtags operated from radiation incident thereupon is that it is difficultto generate potentials on the tags of sufficient magnitude to operatesemiconductor integrated circuits incorporated therein. Such circuitsfrequently require a supply potential of several volts to function.

SUMMARY OF THE INVENTION

The inventor has devised a tag which addresses this principal problemand which is operable, for example, from moderate levels of incidentradiation thereupon in the order of 10 μW. Such moderate levels ofradiation rarely represent any health and safety risk.

According to a first aspect of the present invention, there is provideda piezoelectric tag including receiving means for receiving inputradiation and generating a corresponding received signal, piezo-electricvibrating means for increasing voltage magnitude of the received signalto generate a supply potential and electronic circuit means powerable bythe supply potential.

The invention provides the advantage that the vibrating means is capableof providing voltage magnification, thereby enabling the tag to bepowered from radiation incident thereupon.

For the purpose of describing the invention, microwave frequencies meansfrequencies substantially in a range of 1 GHz to 30 GHz.

Advantageously, the vibrating means comprises a piezo-electrictransformer incorporating mutually vibrationally coupled primary andsecondary regions, the transformer operable to be excited into vibrationby the received signal at the primary region and to generate acorresponding output signal at the secondary region for use ingenerating the supply potential.

The piezo-electric transformer provides the advantage that it is capableof being compact, inexpensive and providing a considerable increase insignal voltage amplitude from its primary region to its secondaryregion, the increase approaching 100 times or more.

Alternatively, the vibrating means comprises a piezoelectric bi-morphoperable to be excited into vibration by the received signal and togenerate a corresponding output signal for use in generating the supplypotential.

As a further alternative, the vibrating means conveniently comprises asilicon micromachined device comprising an array of one or more resonantelements, each element incorporating an associated piezo-electrictransducer operable to generate an element signal in response tovibration of its associated element, the transducers connected in seriesto add their element signals to provide an overall output from which thesupply potential is generated, and driving means operable to be drivenby the received signal for stimulating the one or more elements intovibration and thereby generating the supply potential.

The silicon device provides the advantage that it capable of beingmass-produced and being highly compact, for example 2 mm wide by 2 mmlong by 0.6 mm thick.

Advantageously, the resonant elements in the silicon device are operableat resonance to generate the supply potential. Operation at resonanceprovides the benefit that voltage magnification in the device is greaterthan off-resonance.

Moreover, to obtain even greater voltage magnification, the resonantelements are housed in an evacuated environment. Operation in theevacuated environment increases Q-factor of the resonant elements,thereby increasing voltage magnification provided by the silicon device.

Conveniently, the receiving means in the tag incorporates demodulatingmeans for demodulating modulation components present in the receivedradiation to generate the received signal. Inclusion of the demodulatingmeans provides the benefit of signal frequency transformation, therebyenabling the tag to receive radiation providing power thereto at adifferent carrier frequency to the frequency of vibration required forexciting the vibrating means.

Advantageously, the demodulating means incorporates zero-bias Schottkydiodes for demodulating the received radiation to generate the receivedsignal. The zero-bias Schottky diodes provide the advantage ofexhibiting a smaller forward conduction voltage drop compared to p-nsilicon junction diodes, thereby enabling the tag to function with lowerlevels of received radiation power, for example 10 μW.

Conveniently, the receiving means incorporates one or more conductivemetallic film dipole antennae for one or more of receiving and emittingradiation. Such dipoles provide the advantage of being potentiallycompact and inexpensive to mass-produce.

The tag beneficially incorporates two antennae, one antenna for use ingenerating the received signal and the other incorporated into theresponding means for at least one of emitting and receiving radiation.Incorporating two antennae provides the advantage that each antennae canbe optimized to function at its respective radiation frequency.Conveniently, the antennae are conductive metallic film dipole antennaefor reasons of increased compactness and reduced manufacturing cost.Alternatively, the antennae can also be patch antennae or loop antennae.

In some practical applications of the tag, it is advantageous that thetag is implemented in the form of a block, for example a cuboid block.This form provides the tag with enhanced mechanical robustness andthereby increases its reliability.

When the tag is personnel wearable or attachable to items ofmerchandise, it is convenient that the tag is in the form of a planarcard. This form provides the advantage that the tag can be of similarsize to existing planar cards, for example debit cards, therebyproviding a degree of potential compatibility with existing card readingequipment.

When the tag is implemented in a planar card form, it convenientlyincorporate recesses for accommodating the receiving means, thevibrating means and the responding means. Such recesses provideprotection for the receiving means and the responding means, therebymaking the tag more robust.

In the tag, the circuit means can comprise responding means for emittingoutput radiation from the tag, the responding means powerable by thesupply potential. Incorporation of the responding means enables the tagto be remotely identified when interrogated.

Conveniently, the responding means is a transponder operable to receiveinput radiation to the tag and emit output radiation in response fromthe tag. Incorporation of the transponder enables the tag to beselectively responsive to interrogating radiation in an environmentwhich is flood illuminated with radiation for exciting the vibratingmeans.

Advantageously, the transponder is operable to modulate the outputradiation with a signature code by which the tag can be individuallyidentified. The code enables the tag to be individually recognised whichis highly advantageous where the tag is personnel wearable and used toidentify its wearer, for example as in personal identification tags wornby employees in a commercial establishment.

When operating with high frequency radiation, for example at UHFfrequencies from 300 MHz to 1 GHz and from microwave frequencies from 1GHz to 30 GHz, the tag advantageously has the transponder incorporatinga reflection amplifier for amplifying the input radiation to generatethe output radiation. The reflection amplifier provides the advantagethat it is capable of providing a high gain, for example in a range of+10 dB to +30 dB, for relatively low current consumption, for example inthe order of a few microamperes.

Advantageously, especially when the transponder provides considerablegain, the transponder is operable in a pseudo-continuous mode andincorporates a delay line for delaying the output radiation relative tothe input radiation, thereby counteracting spontaneous oscillation fromarising within the transponder from feedback therein.

Conveniently, the tag is arranged such that the receiving meansincorporates first and second antennae for generating the receivedsignal for exciting the vibrating means, the first antenna adapted torespond to microwave radiation and the second antenna adapted to respondto radiation having a carrier frequency corresponding to a resonantfrequency of the vibrating means. Incorporation of two antennae forgenerating the received signal provides the advantage that the tag ispowerable from radiation having a number of possible carrierfrequencies.

In a second aspect of the invention, there is provided a method ofguiding a vehicle along a path to a destination, the method comprisingthe steps of:

-   (a) distributing a plurality of tags according to the first aspect    along the path and providing the vehicle with a direction sensitive    interrogating source adapted to transpond with the tags;-   (b) interrogating the tags from the source by emitting radiation to    the tags and receiving radiation therefrom, thereby determining    direction of the tags relative to the source and hence determining    the path;-   (c) moving the vehicle along the path; and-   (d) repeating steps (b) and (c) until the vehicle reaches the    destination.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the following diagrams in which:

FIG. 1 is a schematic of a first embodiment of the invention;

FIG. 2: is an exterior perspective view of the first embodiment shown inFIG. 1;

FIG. 3 is an illustration of a second embodiment of the invention;

FIG. 4 is an illustration of a third embodiment of the inventionincorporating a simplified circuit utilising loop antennae;

FIG. 5 is an illustration of a fourth embodiment of the inventionadapted for operating with Manchester encoded signals; and

FIG. 6 is an illustration of a fifth embodiment of the inventionincorporating a single antenna for use in emitting and receivingradiation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, there is shown a piezo-electric tag according to afirst embodiment of the invention indicated by 10. The tag 10incorporates a number of sections, namely a first dipole antennaindicated by 20 and included within a dotted line 22, a firstrectification circuit indicated by 30 and included within a dotted line32, a piezoelectric transformer indicated by 40 incorporating a primaryregion 42 and a secondary region 44, a second rectification circuitindicated by 50 and included within a dotted line 52, and a transpondercircuit indicated by 60 and included within a dotted line 62. Thesections are incorporated into a plastic card having external dimensionsof 55 mm width, 85 mm length and 1 mm thickness; this will be furtherdescribed later with reference to FIG. 2.

The transponder 60 incorporates a dipole antenna indicated by 64 andincluded within a dotted line 66, a bi-directional surface acoustic wave(SAW) delay line 68 and a reflection amplifier 70.

The first dipole antenna 20 is connected to an input of the firstrectification circuit 30. The circuit 30 includes an output which isconnected to the primary region 42 of the transformer 40. The secondaryregion 44 thereof is connected to an input of the second rectificationcircuit 50. The second circuit 50 incorporates an output which isconnected to a power supply input to the transponder 60.

Operation of the tag 10 will now be described in broad overview afterwhich its sections will be described in further detail.

The antenna 20 receives incoming radiation 100 from an interrogatingsource (not shown). The radiation 100 has a carrier frequency of 1 GHzwhich is amplitude modulated to a modulation depth in a range of 50% to100% by a modulating signal which has a frequency of 300 kHz. Moreover,the radiation 100 has a power density of 5 mW/m² at the antenna 20. Theradiation 100 couples to the antenna 20 and generates a correspondingsignal S_(a) across output terminals T₁, T₂ of the antenna 20; thesignal S_(a) has a frequency of 1 GHz and an amplitude in the order of80 mV. The signal S_(a) propagates to the first circuit 30 whichdemodulates it and then filters it to substantially remove signalcomponents above 1 MHz to generate a unipolar modulated signal S_(b)having signal components at 300 kHz. The transformer 40 receives thesignal S_(b) across its primary region terminals P₁, P₂. The signalS_(b) stimulates the primary and secondary regions 42, 44 to resonate at300 kHz in their longitudinal mode of vibration. At resonance, thetransformer 40 magnifies the signal S_(b) received at its primary region42 to generate a bipolar alternating signal S_(c) at a secondary regionterminal S₁, the signal S_(c) having an amplitude in the order of 3volts. The second circuit 50 receives the signal S_(c) and demodulatesand filters it to generate a substantially smoothed unipolar signalS_(d) at an output terminal of the circuit 50. The transponder 60receives the signal S_(d) and uses it as a supply potential to poweractive circuits incorporated thereinto.

The transformer 40 provides the advantage of performing a step-upvoltage conversion function from its primary region 42 to its secondaryregion 44 at resonance, thereby providing the signal S_(d) of sufficientmagnitude of several volts to power active electronic devicesincorporated into the transponder 60, namely the reflection amplifier70. Although the transformer 40 cannot provide power gain, it iseffective to provide an impedance conversion for matching an inputimpedance presented by the second circuit 50 to an output impedancepresented by the first circuit 30; the signal S_(a) of relatively lowervoltage amplitude from the antenna 20 which is unsuitable for poweringcircuits is thereby converted into the signal S_(d) of relatively highvoltage, namely several volts, which is suitable for powering circuits.

The transponder 60 receives incoming continuous-wave radiation 102 fromthe interrogating source. The radiation 102 has a carrier frequency of1.5 GHz. In response to receiving the radiation 102, the antenna 64generates a corresponding signal S_(c) at its terminals which passes tothe delay line 68 and propagates therethrough whilst being delayedtherein to provide a signal S_(f) at an input to the reflectionamplifier 70. The amplifier 70 presents a modulated negative resistanceat its input/output terminal and thereby reflectively amplifies thesignal S_(f) to generate a corresponding modulated amplified signalS_(g). The signal S_(g) propagates back through the delay line 68 whilstbeing delayed therein to the antenna 64 from where it is emitted asreturn radiation. The interrogating source receives the return radiationand determines that it is modulated, thereby detecting the presence ofthe tag 10.

The tag 10 provides the benefit that it is capable of providing themodulated return radiation without there being a need for the tag 10 toincorporate limited lifetime power sources such as batteries forpowering its active circuits. Avoidance of the need for batteriesprovides the tag 10 with a potentially useable lifetime of severaldecades or more. Thus, the tag 10 is thereby suitable for attachment toproducts which ae to be stored for lengthy periods of time, for exampleseveral years.

Sections of the tag 10 will now be described in more detail.

The antenna 20 is a thin film dipole formed by conductive trucks on amajor surface of the card. It is designed to operate at a radiationfrequency of 1 GHz. The terminal T₂ of the antenna 20 is connected to asignal ground on the card, and the terminal T₁ is connected to the firstcircuit 30.

The circuit 30 incorporates two zero-bias Schottky diodes D₁, D₂ and afilter capacitor C₁. The diode D₂ is connected by its anode to the diodeD₁ at its cathode to form an input terminal; the terminal is connectedto the terminal T₁ of the antenna 20. The diode D₂ is connected at itscathode to a first terminal of the capacitor C₁. The capacitor C₁incorporates a second terminal which is connected to the signal ground.The diode D₁ incorporates an anode which is also connected to the signalground.

The diodes D₁, D₂ are operable to provide signal rectification atmicrowave frequencies, for example 1 GHz, and be responsive to signalamplitudes in the order of in mV. They incorporate metal-semiconductorjunctions for performing rectification. Ordinary p-n silicon junctiondiodes are not as desirable for use in substitution for the diodes D₁,D₂ because of their relatively greater voltage drop when operating underforward bias. The capacitor C₁ is operable to shunt signal components atmicrowave frequencies to the signal ground. An output from the circuit30 is extracted from across the capacitor C₁, namely from the firstterminal of the capacitor C₁ relative to the signal ground.

The transformer 40 is fabricated from a had piezoelectric lead zirconatetitanate (PZT) material whose dielectric loss coefficient is less than0.02; the dielectric loss coefficient is defined as a ratio of energydissipated per cycle to energy stored per cycle. It has exteriordimensions of 3 mm width, 6 mm length and 1 mm thickness and istherefore of an elongate form having an elongate axis. In operation, itis designed to periodically vibrate in a longitudinal manner along theelongate axis at a resonant frequency of approximately 300 kHz. Theprimary region 42 comprises a multilayer stack of piezoelectricelements, each element having exterior dimensions of 3 mm length, 3 mmwidth and 0.1 mm thickness and polarised in its thickness direction. Thesecondary region 44 comprises a single element having exteriordimensions of 3 mm width, 3 mm length and 1 mm thickness; the region 44is polarised in a direction parallel to the elongate axis when assembledin the transformer 40. The elements of the primary region 42 and thesecond region 44 are mutually joined by sintering them together or usingan epoxy resin of comparable rigidity to the PZT material.

In operation, the transformer 40 exhibits a longitudinal resonance modeat 300 kHz frequency having an associated Q-factor in the order of 100.It functions at its resonance to magnify the voltage amplitude ofsignals applied to its primary region 42 by generating correspondingsignals at its secondary region 44 of relatively greater voltageamplitude. This magnification arises at the expense of reduced signalcurrent at the secondary region 44 compared to the primary region 42; inother words, the transformer 40 provides an impedance match but does notimpart power gain.

The circuit 50 employs an identical configuration to the civic 30. Thecapacitor C₁ and the diodes D₁, D₂ in the circuit 30 correspond to acapacitor C₂ and diodes D₃, D₄ in the circuit 50 respectively.

The reflection amplifier 70 of the transponder 60 is connected at itspower supply connections to the signal ground and to the first terminalof the capacitor C₂ which is not connected to the signal ground.Electrical power is thereby supplied to the amplifier 70 in operation.

The reflection amplifier 70 incorporates a switching oscillator whichperiodically switches reflective gain provided by the amplifier 70between a high gain state and a low gain state. The oscillator isoperable to switch the amplifier 70 in a cyclical manner between thehigh gain state for a period of 2τ and the low gain state for a periodof 2τ. In the low gain state, the amplifier 70 is incapable ofsustaining spontaneous oscillation within the transponder 60. The periodof 2τ corresponds to twice a time duration for signals to propagate inone direction through the delay line 68. Periodic switching of gainprovided by the amplifier 70 counteracts the formation of spontaneousoscillation within the transponder 60 because amplified signals from theamplifier 70 are reflected from the antenna 64 and return to theamplifier 70 when it is switched to its lower gain state. In its highgain state, the amplifier 70 provides +23 dB gain which could result inthe formation of spontaneous oscillation if the amplifier 70 were notperiodically gain switched to the lower gain state as described above.

Referring now to FIG. 2, there is provided an exterior perspectiveillustration of the tag 10. The tag 10 incorporates a non-conductingplastic substrate layer 200 having first and second major faces. Ontothe first major face is bonded a conductive earth-plane layer 210 ofaluminium material in a range of 30 μm to 100 μm thick. The layers 200,210 have a length of 85 mm in an x-direction indicated by an arrow 212,and a width of 55 mm in a y-direction indicated by an arrow 214. Thelayer 200, 210 have a combined thickness of 1 mm in a z-directionindicated by a arrow 216.

The substrate layer 200 incorporates recesses 230, 240, 250, 260 mouldedthereinto to accommodate the circuits 30, 50, the transformer 40, theamplifier 70 and the delay line 68 respectively. Being elongate, the tag10 has an elongate axis in the x-direction. At first and second elongateends of the tag 10, the are formed the antennae 20, 64 respectively. Theantennae 20, 64 are both bow-tic dipole antennae incorporating depositedmetallic regions formed onto the second major face of the layer 200.Connecting conductive tracks are also formed on the second major face toconnect the antennae 20, 64 to the circuits 30, 50 and the delays line68 respectively. Further tracks are included to connect the circuits 30,50 to the transformer 40 and the amplifier 70, and the delay line 68 tothe amplifier 70. Wire bonding techniques are employed for bonding fromthe tracks to the recesses 230, 240, 250, 260.

When fabricated a 100 μm thick protective plastic layer (not shown) isadded onto the second major face to protect the antennae 20, 64, thetacking, the circuits 30, 50, the transformer 40, the amplifier 70 andthe delay line 68. Graphical information for example optically readablebar codes or a photographic image, can be optionally printed onto theprotective layer. The photographic image is particularly relevant whenthe tag 10 is personnel wearable and used as a remotely interrogatableidentity tag.

Referring now to FIG. 3, there is shown a piezoelectric tag according toa second embodiment of the invention indicated by 300. The tag 300 isidentical to the tag 10 except that it additionally includes a planarcoil 310 in parallel connection with the capacitor C₁.

The earth plane layer 210 can be selectively absent in a vicinity of thecoil 310 so as not to excessively screen the coil 310. The coil 310 isformed onto the second major face of the layer 200 shown in FIG. 2adjacent to the circuits 30, 50 and the transformer 40. The capacitorC₁, in parallel with an electrical capacitance presented by thetransformer 40 between its terminals P₁, P₂, and the coil 310 areoperable to parallel resonate at the resonant frequency of thetransformer 40, namely 300 kHz. Inclusion of the coil 310 enables thetag 300 to be powered not only from 1 GHz radiation received at theantenna 20 but also from inductively coupled magnetic fields at 300 kHzcoupling to the coil 310. The tag 300 can thereby be powered in twodifferent modes so that it can be used in environments where at eitheror both frequencies, 300 kHz and 1 GHz, are present; for example, inenvironments where microwave radiation cannot be tolerated for safetyreasons.

As an alternative to using the diodes D1 to D4 in the tags 10, 300, FETsfunctioning as asynchronous detectors may be employed. FETS operating inthis mode exhibit a voltage drop thereacross in the order of microvolts.

Moreover, the antennae 20, 64 may be substituted by a single patchantenna or a single loop antenna operable to receive and emit radiationand convey signals to the circuit 30, and to and from the delay line 68.Although the tags 10, 300 are described as being receptive and emissiveat radiation frequencies of 1 GHz and 1.5 GHz, they can be operated atother microwave frequencies by modifying dimension of features of theantennae 20, 64 and the delay line 68. At microwave frequencies inexcess of 10 GHz, the delay line 68 is advantageously replaced by amagnetostatic wave delay line (MWDL), for example a delay lineincorporating a film of yttrium iron garnet (YIG) providing a signalpropagation path in the delay line.

Furthermore, the tags 10, 300 can be modified by replacing thetransponder 60 with, for example, a simple oscillator emitting throughits antenna encoded radiation unique to the oscillator, thereby enablingthe tags 10, 300 when modified to be uniquely identified from theradiation emitted therefrom. Additionally, the transponder 60 can beoperable to emit radiation during a first period and be inactive duringa second period, the transponder arranged to switch cyclically betweenthe first and second period; this provides the advantage that thetransponder 60 can respond by emitting bursts of relatively morepowerful radiation during the first period and conserve energy duringthe second period.

Referring now to FIG. 4, is an illustration of a piezoelectric tagindicated by 400 which incorporates a simplified circuit utilising afirst loop antenna 410 for receiving radiation, a transmitter module(TX) 420 and a second loop antenna 430 for emitting radiation. The tag400 further comprises the transformer 40 and the second rectificationcircuit 50. In a similar manner to the tags 10, 300, the tag 400 ispowered from radiation incident thereupon.

The antenna 410 includes first and second connections, the fastconnection connected to a signal earth plane of the tag 400 and thesecond connection connected to the terminal P₁ of the transformer 40.The terminal P₂ of the transformer 40 is connected to the signal earthplane. The terminal S₁ of the transformer 40 is connected to the circuit50, and the output from the circuit 50 is connected to a V_(S) powerinput of a pulsed transmitter 420. The transmitter 420 is also connectedto the signal earth plane. Moreover, the transmitter 420 includes anoutput Q which is connected to a first connection of the antenna 430. Asecond connection of the antenna 430 is connected to the signal earthplane.

The antenna 410 provides an inductance at its connections which isarranged to electrically resonate with a capacitance exhibited by thetransformer 40 across its terminals P₁, P₂ at a frequency correspondingto input radiation to the tag 400 and also to a vibrational mode of thetransformer 40 when functioning to increasing signal voltage from itsprimary region to its secondary region. The transmitter 420 incorporatesa transistor biased into class C mode of operation such that it onlyconducts for part of a signal cycle when functional when an output fromthe circuit 50 to the transmitter 420 exceeds a threshold value. Whenthe output from the circuit 50 is less than the threshold value, thetransistor is non-conducting, thereby conserving power and providing thecircuit 50 with maximum opportunity to develop a potential.

Operation of the tag 400 will now be described with reference to FIG. 4.The antenna 410 receives radiation incident on the lag 400 at afrequency of 300 kHz and provides a 300 kHz signal across the terminalsP₁, P₂ which excites the transformer 40 into resonance. The transformer40 provides a voltage stepped-up signal at a frequency of 300 kHz at itssecondary terminal S₁. The signal passes to the circuit 50 whichrectifies it to provide a d.c. potential across the capacitor C₂. Thispotential is supplied to the transmitter 420 at its V₁ power input. Whenthe potential exceeds a value of 2 volts relative to the signal earth,the transmitter 420 becomes active and generates at its output Q anoutput signal in the form of busts of signal, each burst comprising asequence of 500 kHz pulses, each burst having a duration of 50 μsec andthe bursts having a repetition ate of 2 Hz. The output signal couplesfrom the transmitter 420 to the antenna 430 from where it is emitted asradiation.

The tag 400 provides the advantage that it is simpler and potentiallycheaper to manufacture than the tags 10, 300. When the tag 400 ismanufactured in volume, the transmitter 420 of each tag 400 can becustomized to generate bursts of 500 kHz radiation at a repetition rateunique to the tag 420, thereby distinguishing it from other tags ofidentical design. Class C operation provides the advantage that thetransistor does not consume power until radiation above a thresholdamplitude is received at the tag 400 which causes the transistor to bedriven into an active region of its characteristics.

Modifications can be made to the tag 400 without departing from thescope of the invention. For example, the transformer 40 can be replacedby a piezoelectric vibrating bi-morph or a silicon micromachinedvibrating structure capable of providing an increased signal voltage atits secondary region relative to its primary region.

Referring now to FIG. 5, there is shown a tag indicated by 500 foroperating with Manchester bi-phase encoded signals. The tag 500comprises the antenna 20, the circuits 30, 50, and the transformer 40.It further comprises a logic unit 510 and a transmitter 520 linked to aloop antenna 530. The antenna 20 is connected to the circuit 30 which isin turn connected to the transformer 40 and then to the circuit 50 in anidentical manner to the tag 10. An output from the circuit 50 generatedacross the capacitor C₂ is connected to the logic unit 510 and thetransmitter 520. Inputs Clk and “Data input” of the unit 510 areconnected to the terminals S₁ and P₁ of the transformer 40 respectively.

The unit 510 incorporates an output D_(o) which is connected to an inputD₁ of the transmitter 520. The transmitter 520 includes an output Uwhich is connected to one connection of the antenna 530; anotherconnection of the antenna 530 is connected to a signal earth of the tag500.

A Manchester bi-phase encoded signal M will now be described. A digitaldata signal D has two states corresponding to logic 0 and logic 1. Thesignal D switches between these two states to convey a stream of datacomprising 0's and 1's. The signal D remains in either of the two statesfor periods of not less than 2τ where τ is a time constant. The signal Dis then exclusive-ORed with a clock signal K having a frequency of 1/2τto generate the signal M. The advantage of the Manchester bi-phasesignal is that it is constantly changing even when the signal is in aconstant 0 or 1 state.

Operation of the tag 500 will now be daubed decoding the signal M.Radiation having a carrier frequency of 1 GHz and modulated by thesignal M is received at the antenna 20 which generates a corresponding 1GHz modulated signal. The circuit 30 demodulates the 1 GHz signal togenerate the signal M at the terminal P₁ of the transformer 40. Theclock signal K is arranged to have a principal frequency componentcorresponding to a resonance mode of the transformer 40 at which itprovides voltage increase from its primary region 42 to its secondaryregion 44. Because the transformer 40 exhibits a relatively narrowresonance peak, it is effective at stripping out the signal D from thesignal M to output predominantly the signal K at the terminal S₁. Thesignal at the terminal S₁ then passes to the circuit 50 which rectifiesit to generate a d.c. potential across the capacitor C₂. The potentialpasses to power supply inputs V_(S) of the unit 510 and the transmitter520 to apply power thereto. The signal M present at the terminal P1 andthe signal K present at the terminal S₁ are also conveyed to the inputsClk and “Data input” respectively of the unit 510 which performs anexclusive-OR function to recover the signal D which is then output atthe output D_(o). The signal D propagates from the unit 510 to thetransmitter 520 which is controlled by data conveyed in the signal D.The transmitter 520 responds to the data by emitting modulated 1 MHzradiation from the antenna 530.

The tag 500 provides the advantage that the transformer 40 performs adual function, namely to generate a supply potential to power the tag500 and also to provide signal filtration.

In order to reduce manufacturing cost and increase compactness, theinventor has appreciated that it is desirable that a tag should onlyincorporate a single antenna for both receiving and emitting radiation.In FIG. 6, there is shown a tag indicated by 600 incorporating theantenna 20 and operable to both emit radiation therefrom and receiveradiation thereat. The tag 600 further comprises the circuits 30, 50,the transformer 40 and a transmitter (TX) 610. The terminals T₁, T₂ ofthe antenna 20 are connected to an input to the circuit 30 and to asignal earth respectively. An output from the circuit 30 is connected tothe terminal P₁ of the transformer 40. The terminal P₂ of thetransformer 40 is connected to the signal earth. An output B of thetransmitter 610 is connected through a resistor R₁ to the terminal P₁ ofthe transformer 40. The secondary terminal S₁ of the transformer 40 isconnected to the circuit 50 in a similar manner to the tag 10. Moreover,the transmitter 610 further comprises an output V which is coupledthrough a capacitor C₃ to the terminal T₁ of the antenna 20.

Operation of the tag 600 will now be described with reference to FIG. 6.Initially, the transmitter 610 is not energised such that its output Bis at a potential of the signal earth. Radiation having a carrierfrequency of 1 GHz and modulated with a signal of 300 kHz is received atthe antenna 20 which generates a corresponding signal across itsterminals T₁, T₂. The signal is rectified to generate a 300 kHz signalacross the capacitor C₁ which then passes to the primary region 42 ofthe transformer 40 to excite it into resonance. The transformer 40generates a voltage-enhanced output signal at a frequency of 300 kHz atthe terminal S₁ which is subsequently demodulated by the circuit 50 toprovide a potential for operating the transmitter 610.

The transmitter 610 functions to generate 100 μsec duration bursts of 1GHz signal at a repetition rate of 2 Hz at its output V. When thetransmitter 610 is about to emit a burst of 1 GHz radiation from theantenna 20, it firstly switches its output B to a potential approachingthat supplied by the circuit 50 which reverse biases the diodes D₁, D₂thereby disabling the circuit 30. The transmitter then outputs a burstsignal through the capacitor C₃ to the antenna 20 from whence it isradiated as radiation. At the end of the burst signal, the transmitterswitches its output B back to a potential of the signal earth so thatthe circuit 30 can continue to function to keep the capacitor C₂ chargeduntil a next burst of radiation is to be emitted.

The tag 600 provides a further advantage that, because only one antenna20 is required, the antenna 20 can, if required, be enlarged to occupy amajority of a major surface area of the tag 600. Such enlargement is notpossible to achieve when two or more antennae are incorporated into atag, each antenna requiring more than 50% of the major surface area ofthe tag 600.

It will be appreciated by one skilled in the art that modifications canbe made to the tags 10, 300, 400, 500, 600 without departing from thescope of the invention.

For example, the tags 10, 300, 400, 500, 600 can be moulded into aplastic block rather than being implemented in card-like form asillustrated in FIG. 2. The block is a more robust shape compared to acard, thereby enable the tags 10, 300, 400, 500, 600 in block form to bedeployed in rugged environments, for marking out a path in asmoke-filled burning building. A block is distinguishable from a card inthat the ratio of the block's length, width and thickness dimensions areless than 1:3. A block form also includes a cuboid form, a pyramidalform and a near-spherical or spherical form.

As an alternative to using the diodes D1 to D4 in the tags 10, 300, 400500, 600, FETs functioning as asynchronous detectors may be employed.FETs operating in this mode exhibit a voltage drop thereacross in theorder of microvolts which is less than a forward bias voltage dropassociated with diodes.

The tags 10, 300, 400, 500, 600 can be used as personnel wearableidentity tags. They may be attached to items of merchandise and used inconjunction with an associated interrogating source to provide amerchandise anti-theft system.

The tags 10, 300, 400, 500, 600 can be used in a similar manner to magiceye reflectors used to delineate lanes on motorways; a plurality of thetags 10, 300, 400, 500, 600 can be employed as interrogatable markersfor marking out a path. Such use is potentially valuable, for example,for defining routes for automatically guided robotic vehicles aroundmanufacturing and storage sites. The guided vehicles can be equippedwith interrogating sources which are sensitive to direction of radiationemitted from the tags 10, 300, 400, 500, 600 thereby defining directionof the tags 10, 300, 400, 500, 600 relative to the vehicles. Each tag10, 300, 400, 500, 600 can be provided with its own unique signaturecode, thereby enabling the vehicle to determine its position along thepath from the signature codes. Such a method of vehicle guidance ispreferable to wire guided vehicle systems where greater installationcost can arise when installing guiding wires compared to distributingtags.

In the tags 10, 300, 400, 500, 600, the transformer 40 can be replacedby an alternative piezo-electric device operable to increase voltage.One example of an alternative piezo-electric device is a ceramicbi-morph in the form of an elongate member supported at one of its endand free the vibrate at its other end; such a bi-morph is capable ofexhibiting a higher Q-factor than the transformer 40, thereby providingan enhanced voltage increase. Another example of an alternativepiezo-electric device is a micromachined silicon device comprising anarray of one or more suspended silicon cantilevers, each cantileverincorporating a deposited film piezo-electric transducer operable togenerate a signal in response to vibration of the cantilever. Thetransducers are connected in series to add their signal voltagestogether to provide an overall output for the circuit 50. An excitationtransducer operable to be driven by a drive signal from the circuit 30is also incorporated for mechanically exciting the one or morecantilevers into vibration, preferable at resonance of the cantilevers.Silicon cantilevers are capable of exhibiting high resonance Q-factorsapproaching several million when operating in a miniature evacuatedhousing, thereby providing a considerable increase in signal voltageamplitude at the overall output compared to the drive signal. Siliconmicromachining is a well known mass production press and involvesfabrication of mechanical structures in silicon material using batchlithographic, deposition and etching techniques.

The tags 10, 300, 400, 500, 600 can be modified to include other typesof electronic circuits, for example memory circuits and environmentalsensors, for example radiation and chemical sensors. Such electroniccircuits enable the tags to function as miniature personal data loggerswhich are personnel wearable and useable for monitoring the safety ofpersonnel in working environments, for example in chemical laboratorieswhere hazardous chemicals are handled.

The tags 10, 300, 400, 500, 600 can be further miniaturised and adaptedfor inclusion within biological systems, for example for use as remotelycontrolled insulin dispensers, as heart-stimulating pace-makers or asartificial retina. Use of piezo-transformers powered from receivedmodulated radiation avoids the need for batteries in the tags andthereby enables the tags to be implanted permanently within biologicalsystems without needing to be periodically removed.

1. A piezo-electric tag, comprising: a) receiving means for receiving input radiation and generating a corresponding received signal; b) piezo-electric vibrating means for increasing voltage magnitude of the received signal to generate a supply potential; and c) electronic circuit means powerable by the supply potential.
 2. The tag according to claim 1, wherein the vibrating means comprises a piezo-electric transformer incorporating mutually vibrationally coupled primary and secondary regions, the transformer being operable to be excited into vibration by the received signal at the primary region and to generate a corresponding output signal at the secondary region for use in generating the supply potential.
 3. A tag according to claim 1, wherein the vibrating means comprises a piezo-electric bi-morph operable to be excited into vibration by the received signal and to generate a corresponding output signal for use in generating the supply potential.
 4. The tag according to claim 1, wherein the vibrating means comprises a silicon micromachined device comprising an array of resonant elements, each element incorporating an associated piezo-electric transducer operable to generate an element signal in response to vibration of its associated element, the transducers being connected in series to add their element signals to provide an overall output from which the supply potential is generated, and driving means operable to be driven by the received signal for stimulating the elements into vibration and thereby generating the supply potential.
 5. The tag according to claim 4, wherein the resonant elements are operable at resonance to generate the supply potential.
 6. The tag according to claim 4, wherein the resonant elements are housed in an evacuated environment for increasing their resonance Q factor.
 7. The tag according to claim 1 wherein the receiving means incorporates demodulating means for demodulating modulation components present in the received radiation to generate the received signal.
 8. The tag according to claim 7, wherein the demodulating means incorporates zero-bias Schottky diodes for demodulating the received radiation to generate the received signal.
 9. The tag according to claim 7, wherein the demodulating means incorporates transistors operable as synchronous demodulators for demodulating the received radiation to generate the received signal.
 10. The tag according to claim 1, wherein the circuit means is operable to function in a class C mode for reducing tag power consumption.
 11. The tag according to claim 1, wherein the receiving means incorporates first and second antennas for generating the received signal for exciting the vibrating means, the first antenna being adapted to respond to microwave radiation, and the second antenna being adapted to respond to radiation having a carrier frequency corresponding to a resonant frequency of the vibrating means.
 12. The tag according to claim 1, wherein the receiving means incorporates at least one of a metallic film dipole antenna, a loop antenna and a patch antenna for at least one of receiving and emitting radiation.
 13. The tag according to claim 1, wherein the circuit means comprises responding means for emitting output radiation from the tag, the responding means being powerable by the supply potential.
 14. The tag according to claim 13, wherein the vibrating means is operable to recover a clock component of Manchester bi-phase encoded radiation received at the tag, and wherein the responding means is operable to use the clock component to demodulate the encoded radiation to generate corresponding demodulated data for use in the tag.
 15. The tag according to claim 13, wherein the tag incorporates two antennas, one antenna for use in generating the received signal, and the other antenna incorporated into the responding means for at least one of emitting and receiving radiation.
 16. The tag according to claim 13, wherein the antennas are conductive metallic film dipole antennas.
 17. The tag according to claim 1, the tag having a form of a block.
 18. The tag according to claim 1, the tag having a form of a planar card.
 19. The tag according to claim 18, wherein the card incorporates recesses for accommodating the receiving means, the vibrating means and the responding means.
 20. The tag according to claim 13, wherein the responding means is a transponder operable to receive input radiation to the tag and emit output radiation in response from the tag.
 21. The tag according to claim 20, wherein the transponder is operable to modulate the output radiation with a signature code by which the tag is individually identified.
 22. The tag according to claim 20, wherein the transponder incorporates a reflection amplifier for amplifying the input radiation to generate the output radiation.
 23. The tag according to claim 20, wherein the transponder is operable in a pseudo-continuous mode and incorporates a delay line for delaying the output radiation relative to the input radiation, thereby counteracting spontaneous oscillation from arising within the transponder from feedback therein.
 24. The tag according to claim 1, and a metallic earthing plane for providing a common signal earth for the tag.
 25. The tag according to claim 1, and means for implantation into a biological system and operable for at least one of monitoring and stimulating the biological system. 